In recent years, the amount of transferred information has been dramatically increased with the spread of an optical fiber communication system. With the increase of the amount of transferred information, high-capacity long-distance optical communication is performed using a large number of optical fibers such as a dozen or a hundred of optical fibers in the optical fiber communication system.
To increase a transmission capacity per one optical fiber in such an optical fiber communication system, it is known that a multi-core fiber in which outer peripheries of a plurality of cores are covered with a single clad is used and a plurality of signals is transferred by light propagating in each of the cores. Also, in the optical fiber communication system, there is a case where a plurality of optical fibers is connected and used when the long-distance optical communication is performed. There is also a case where a plurality of multi-core fibers is connected and used when a multi-core fiber is used.
In connecting optical fibers, end surfaces of respective optical fibers are sometimes butted against each other using discharge welding. In discharge welding, usually, a portion to be welded, which the end surfaces of the optical fibers to be connected are butted against each other, is arranged between a pair of electrodes, and discharge is carried out between the pair of electrodes, so that the end surfaces of the optical fibers are fused (see Patent Document 1). Even in a case where the multi-core fibers are connected, positions of cores of the multi-core fibers to be connected are adjusted, so that the end surfaces of the multi-core fibers may sometimes be fused by discharge welding.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-3170